Cardiovascular disease continues to be a leading cause of mortality and morbidity in the US. Coronary vascular disease and hypertension induce damage of the heart. In particular, the heightened stress state either acutely or over time, induces myocyte death. This promotes dysfunctional remodeling of the cardiac chambers and depression of cardiac pump performance. A final common pathway for progressive depression of heart function in heart failure is the death of cardiac myocytes. Therefore, therapies that would restore myocyte number are being evaluated in many laboratories. The field of stem cell mediated cardiac repair has exploded over the past few years, with a number of preclinical studies and early clinical trials showing improvement in cardiac function after cell therapy. The mechanism underlying improved cardiac function after cell therapy is controversial with some showing stem cell mediated myocyte regeneration and others suggestive of paracrine effects. An often ignored factor is that damaged cardiac myocytes in diseased hearts uncouple from their neighbors. This effect is most pronounced at the border of ischemic regions of a myocardial infarction. Our hypothesis is that adult stem/progenitor cells with the ability to differentiate into cardiac myocytes must electrically couple to myocytes in order to differentiate and mature. If true, the inability of damaged myocytes to couple to either resident or injected stem cells will limit cardiac regeneration. The aims of the current proposal are to 1: test the idea that adult cardiac derived stem/progenitor cells must electrically couple to myocytes (cultured neonatal rat ventricular myocytes) in order to differentiate into fully functional cardiac myocytes; 2: define the respective contributions of calcium influx through T- and L-type Ca channels and cyclic changes in Ca coming into newly forming myocytes from Ca transients in coupled myocytes to stem cell differentiation; and 3: determine if enhancing the coupling between injected stem/progenitor cells and cardiac myocytes in damaged hearts will increase cardiac regeneration and improve cardiac function. Our study will provide fundamental new knowledge about the role of calcium cycling and myocyte phenotypes as determinants of new myocyte formation in the intact heart. Our ultimate goal is to identify approaches that will enhance cardiac regeneration after injury. Our preclinical studies in animal models will apply these strategies to determine if they might be useful for treating patients with many different forms of cardiac injury.